Mammography has long been used to screen for breast cancer and other abnormalities. Traditionally, mammograms have been formed on x-ray film. More recently, flat panel digital imagers have been introduced that acquire a mammogram in digital form, and thereby facilitate analysis and storage of the acquired images, and provide other benefits as well. Further, substantial attention and technological development has been dedicated towards obtaining three-dimensional images of the breast, using methods such as breast tomosynthesis. In contrast to the 2D images generated by legacy mammography systems, breast tomosynthesis systems construct a 3D image volume from a series of 2D projection images, each projection image obtained at a different angular displacement of an x-ray source relative to the image detector as the x-ray source is scanned over the detector. The constructed 3D image volume is typically presented as a plurality of slices of image data, the slices often being geometrically reconstructed on planes parallel to the paddle, although other reconstruction angles are possible. The reconstructed tomosynthesis slices reduce or eliminate the problems caused by tissue overlap and structure noise present in single projection 2D mammography imaging, by permitting a reviewer (e.g., a radiologist or other medical professional) to scroll through the image slices to view underlying structures.
Tomosynthesis systems have recently been developed for breast cancer screening and diagnosis. In particular, Hologic, Inc. (www.hologic.com), has developed a fused, multimode mammography/tomosynthesis system that acquires one or both types of mammogram and tomosynthesis images, either while the breast remains immobilized or in different compressions of the breast. Other companies have proposed the introduction of systems which are dedicated to tomosynthesis imaging only, i.e., which do not include the ability to also acquire a 2D mammogram.
However, systems restricted to tomosynthesis acquisition and image display are slow to gain acceptance as a replacement for conventional 2D mammogram images. In particular, conventional 2D mammograms provide good visualization of micro-calcifications, and can offer higher spatial resolution, when compared with tomosynthesis images. While tomosynthesis images provided by dedicated breast tomosynthesis systems have many desirable characteristics, e.g., better isolation and visualization of structures in the breast, such systems do not necessarily leverage existing image interpretation expertise.
In particular, because of the limited angle employed during tomosynthesis image acquisition, a breast structure would normally be visible on multiple tomosynthesis reconstructed image slices. However, it is only near the actual “depth” (location along the z-axis of the tomosynthesis images) where the breast structure is actually located that the image slices for that structure will provide sharp margin/contour/detail of the structure, i.e., as if the structure/object is “in-focus”; whereas on other slices, the structure/object may be visible but associated with fuzzy margin/contour/detail, i.e., as if the structure/object is “out-of-focus.” Further, it is possible that some objects or regions of interest will only be recognizable in image slices that are reasonably close to the actual object/structure depth. As such, a reviewer may need to expend a relatively significant amount of time navigating through the images of the entire breast tomosynthesis stack, which can typically include 40-100 images, or more, depending on the breast size and reconstruction slice thickness, in order to locate a “best focus” image of an object or region of clinical interest for purposes of evaluation. This additional time needed to review a tomosynthesis stack can detour the reviewer from otherwise taking full advantage of the additional benefits provided by detailed tomosynthesis image slices over a traditional mammogram, especially when given the limited amount of time typically allocated for the review.
Thus, it would be of particular benefit to provide a system and methods for providing the reviewer with the ability to quickly and accurately locate an image or subset of images having a best focus of an object or region of interest in a tomosynthesis stack.